Conventionally, it is known that a semiconductor material that functions as an optical semiconductor decomposes water into hydrogen and oxygen when the semiconductor material is irradiated with light (see, for example, Patent Literature 1). Patent Literature 1 discloses a technique in which an n-type semiconductor electrode and a counter electrode are disposed in an electrolyte and the surface of the n-type semiconductor electrode is irradiated with light, so that hydrogen and oxygen are obtained from the surfaces of both electrodes. Specifically, the use of a TiO2 electrode or the like as the n-type semiconductor electrode is described therein.
However, the band gap of TiO2 (anatase type) is 380 nm. Therefore, the semiconductor electrode disclosed in Patent Literature 1 has a problem in that only about 1% of sunlight can be utilized.
In order to solve this problem, Patent Literature 2 discloses an electrode made of a single crystal InzGa1-zN (0<z<1), which is a solid solution of GaN (band gap: 365 nm, crystal structure: wurtzite type) and InN (crystal structure: wurtzite type). A gas generator disclosed in Patent Literature 2 uses the single crystal InzGa1-zN to narrow the band gap of the electrode material, that is, to increase the sunlight utilization efficiency. However, In0.2Ga0.8N (band gap: 500 nm) is the upper limit to which the content of In can be increased in InzGa1-zN. When z is greater than 0.2 in InzGa1-zN, phase separation occurs. Therefore, it is difficult to further reduce the band gap of InzGa1-zN by increasing the content of In.
As described above, it is difficult to achieve an In-rich composition (a composition with a high content of In) in InzGa1-zN. On the other hand, it is possible to achieve an In-rich composition in an oxide semiconductor containing Ga and In. Patent Literature 3 discloses InGaO3(ZnO)m (where m is an integer of 1 to 20) as an oxide semiconductor having an In-rich composition.